Motion of microscopic and macroscopic moving devices, in particular over highly steep or even vertical surfaces or even in absence of gravity, is widely studied in robotics. These extreme situations require moving devices capable to adhere to the rolling surface, so as to prevent them from slipping thereon or detaching therefrom in case of vertical surfaces, while at the same time preventing them from remaining stuck on the surface.
Magnetic adhesion apparatuses could be realized if the surface on which the moving device should move is metallic, though they would be completely useless in devices adapted to move on surfaces of a different kind. In nature it has been noticed that geckos are capable of climbing walls made of any kind of material. This extraordinary ability is explained in physics with the intermolecular forces of Van der Waals generated between the end terminations of geckos hair and generic surfaces. In particular, each square millimeter of the skin of the fleshy parts of the fingertips of geckos have thousands of keratin bristles (setae). Each bristle has a diameter of about half a micrometer and fringes at the tip in hundreds of branches (spatula), each of which has a size in the order of a nanometer. The adhesion onto the surfaces is thus due to intermolecular Van der Waals forces, that is very weak electrostatic attraction forces distributed among molecules and that, multiplied by the very large contact surface due to the extreme density and ramification of the setae, determine a great adhesion force.
When geckos want to stick to a surface, they flatten the fleshy parts of the fingertips such to obtain the maximum contact surface of the setae. For moving, before lifting a leg, geckos incline their fingers to modify the angle of the spatulae with respect to the bearing plane. In so doing, the contact surface between the setae and the bearing plane decrease until a “critical level” is attained, below which adhesion forces abruptly drop.
Researches carried out on geckos have been published in the article “Nanotubes Adhesive Sticks Better Than A Gecko's Foot”, ScienceDaily, Jun. 20, 2007, wherein the scientists of the Rensselaer Polytechnic Institute and of the University of Akron affirm that they realized a very strong adhesive ribbon made of carbon nanotubes, that uses the typical adhesion forces of geckos setae. The authors of this article sustain that the ribbon may be attached and detached on a great variety of materials, including Teflon™, that the ribbon has adhesion characteristics up to ten times larger than those of geckos, and that the ribbon is capable of generating anisotropic cohesion forces, i.e. that vary depending on the considered direction. In particular, the generated adhesion force ranges from 10 N (the force naturally present between geckos fingers and walls) up to 100 N. The function of nanotubes is essentially to increase the contact surface.
The detachment is made possible because the setae are realized on a flexible substrate, thus it is possible to forcibly modify the inclination of setae to nullify the Van der Waals forces between setae and adhesion surfaces.
Even if in this article the use of the adhesive layer is proposed for realizing feet of robots capable of climbing vertical walls, nothing is said about how and with which power dissipation a robot could move if its feet adhere to the wall with an adhesion force sufficient to withstand at least its weight.